March 30 (Reuters) - BP Plc has accused the U.S.
government of withholding evidence that may show the 2010
Deepwater Horizon oil spill in the Gulf of Mexico was smaller
than federal officials claimed, a key issue in determining the
oil company's liability.

A reduction in the size of the spill would lower the maximum
civil fine BP could be forced to pay under the U.S. Clean Water
Act, a sum now estimated as high as $17.6 billion.

The government is one of many plaintiffs suing BP over the
April 20, 2010 explosion of the Deepwater Horizon drilling rig,
which killed 11 workers and triggered the largest U.S. offshore
oil spill.

In a filing late on Thursday with the U.S. district court in
New Orleans, BP said more than 10,000 documents the government
is refusing to turn over "appear to relate to flow rate issues"
at the company's ruptured Macondo well.

BP said the documents, which the government considers
privileged because they reflect policy deliberations, may show
that an August 2010 estimate that 4.9 million barrels of oil
spilled from the well is too high.

"The United States' invocation of the deliberative process
privilege here sweeps too broadly," because it shields evidence
concerning "a factual issue, namely, the amount of oil
discharged," wrote Don Haycraft, a lawyer for BP.

"Fundamental fairness" requires that BP get access to this
evidence for its defense, he added.

Wyn Hornbuckle, a U.S. Department of Justice spokesman,
declined to comment.

In an order dated Friday, U.S. Magistrate Judge Sally
Shushan directed BP and the government to meet over the next
couple of weeks to try to resolve disagreements over some of the
challenged evidence.

The Clean Water Act calls for maximum fines of $1,100 per
barrel of oil spilled or $4,300 if there were gross negligence.

Assuming 4.1 million barrels were spilled and not cleaned up
as the government contends, BP could face a maximum $17.6
billion fine if there was gross negligence.

BP agreed in principle on March 2 to pay $7.8 billion to
settle claims by more than 100,000 private plaintiffs for
economic, property and other damages.

It still faces claims from the government, Gulf Coast states
and drilling partners Transocean Ltd and Halliburton
Co.

BP has calculated its legal and cleanup costs to be roughly
$43 billion. The company is based in London.

The settlement with private plaintiffs put a potentially
year-long trial over the spill on indefinite hold. The size of
the spill was among the issues to be determined.

U.S. District Judge Carl Barbier has scheduled a May 3
meeting with lawyers to discuss how the case should proceed.

The case is In re: Oil Spill by the Oil Rig "Deepwater
Horizon" in the Gulf of Mexico, on April 20, 2010, U.S. District
Court, Eastern District of Louisiana, No. 10-md-02179.
(Reporting By Jonathan Stempel in New York; Editing by Andre
Grenon and Steve Orlofsky)